The present invention relates to the field of communications and more particularly to radiotelephone communications.
Cellular communications systems are commonly employed to provide voice and data communications to a plurality of mobile units or subscribers. Analog cellular systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. Digital cellular systems such as designated IS-136 in North America and the pan-European GSM system have also been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993.
As illustrated in FIG. 1, a cellular communication system 20 as in the prior art includes one or more mobile stations or units 21, one or more base stations 23 and a mobile telephone switching office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may comprise hundreds of base stations, thousands of mobile stations and more than one MTSO. Each cell will have allocated to it one or more dedicated control channels and one or more voice channels. A typical cell may have, for example, one control channel, and 21 voice/data, or traffic, channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information.
The MTSO 25 is the central coordinating element of the overall cellular network 20. It typically includes a cellular processor 28, a cellular switch 29 and also provides the interface to the public switched telephone network (PSTN) 30. Through the cellular network 20, a duplex radio communication link 32 may be effected between two mobile stations 21 or, between a mobile station 21 and a landline telephone user 33. The function of the base station 23 is commonly to handle the radio communication with the mobile station 21. In this capacity, the base station 23 functions chiefly as a relay station for data and voice signals. The base station 23 also supervises the quality of the link 32 and monitors the received signal strength from the mobile station 21.
Frequency reuse (Frequency Division Multiple Access or FDMA) is commonly employed in cellular technology wherein groups of frequencies are allocated for use in cells which define regions of limited geographic coverage. Cells containing equivalent groups of frequencies are geographically separated to allow mobile units in different cells to simultaneously use the same frequency without interfering with each other. By so doing many thousands of subscribers may be served by a system of only several hundred frequencies.
In the United States, for example, Federal authorities have allocated to cellular communications a block of the UHF frequency spectrum further subdivided into pairs of narrow frequency bands called channels. Channel pairing results from the frequency duplex arrangement wherein the transmit and receive frequencies in each pair are offset by 45 MHz. At present there are 832, 30-KHz wide, radio channels allocated to cellular mobile communications in the United States.
To address the capacity limitations of this analog system, a digital transmission standard has been provided, designated IS-136, wherein these frequency channels are further subdivided into time slots. The division of a frequency into a plurality of time slots wherein a channel is defined by a frequency and a time slot is known as Time Division Multiple Access (TDMA). Accordingly, multiple channels can be defined on a single frequency.
More recently, Code Division Multiple Access (CDMA) standards have been developed wherein a communications channel is defined by a carrier frequency and a pseudonoise (PN) sequence. Accordingly, multiple communications channels can be provided on a common carrier frequency at the same time wherein each channel on the carrier frequency has a different PN sequence. For example, a CDMA standard for a cellular radiotelephone system is provided by the North American Code Division Multiple Access (CDMA) Digital Cellular (IS-95) standard.
In any of the cellular radiotelephone systems and/or standards discussed above, a user terminal such as a radiotelephone may first need to acquire a channel before communications services can be provided by the cellular radiotelephone system. The acquisition of a channel may require the user terminal to scan a plurality of channels to find a channel that can be used. This scan may result in a delay before communications services are provided. This delay may be especially significant in a CDMA radiotelephone system where scanning for a channel requires first scanning for an available carrier frequency and then scanning for an available PN sequence within the carrier frequency.
In an IS95 CDMA radiotelephone system, for example, each channel on a carrier frequency is encoded with a PN sequence having a length of 32768 bits. Each of the base stations within a system can use the same carrier frequency and the same PN sequence shifted by an integer multiple of 64 bits and this shift is known as a PN sequence offset. The combination of the carrier frequency, the PN sequence, and the PN sequence offset uniquely identify the base station/cell being monitored.
When attempting to acquire a CDMA channel, the user terminal may first scan a specified list of carrier frequencies in the order specified by Section 6.1.1.1 of the IS-95 specification shown below in Table 1.
Upon finding a carrier frequency, the user terminal then searches for one of 512 possible PN sequences on that carrier frequency. If a valid PN sequence is not found on the carrier frequency, the user terminal then scans to find a next carrier frequency which is then searched for a PN sequence. This process may continue through the entire spectrum of carrier frequencies and PN sequences until an available channel is found. Obtaining a channel in a CDMA system can thus consume an excessive amount of time because of the need to tune to new carrier frequencies and search PN space for each carrier frequency.
Accordingly, there continues to exist a need in the art for improved methods of searching and acquiring communications channels.
It is therefore an object of the present invention to provide improved radiotelephone communications methods and terminals.
It is another object of the present invention to provide improved methods of scanning for an available channel and related terminals.
It is still another object of the present invention to provide methods which reduce the time to scan for an available channel and related terminals.
These and other objects are provided according to the present invention by storing a previously acquired communications channel in user terminal memory, searching for the previously acquired communications channel in the user terminal memory, and acquiring the previously acquired communications channel when the previously acquired communications channel is available. Because radiotelephone terminals are often used at a relatively high frequency within a relatively small number of service areas (cells), the communications channels associated with those most frequently used cells may have the highest probability of being available for use.
In other words, the user terminal can first search for channels which are most likely to be found before performing an exhaustive search of all possible channels thereby reducing the time required to acquire a channel for a significant number of communications. If the previously acquired channel is not available, the user terminal can search a list of specified channels with little added delay. Furthermore, the user terminal can store a plurality of previously acquired communications channels in memory so that the searching step comprises searching for the previously acquired communications channels.
In addition, the user terminal can store an access count for each of the previously acquired communications channels wherein the access count identifies a number of times the respective communications channel has been acquired by the user terminal. The searching step can thus include sequentially searching the previously acquired communications channels in order of a highest access count. In other words, those channels with the highest access counts are assumed to have the highest probability of being acquired, and these channels are thus searched first.
The user terminal can alternately identify a last acquired one of the previously acquired communications channels, and then first search for the last acquired communications channel. Here, it is assumed that the last acquired channel is the most likely channel to be acquired. Furthermore, the last acquired channel can be searched first followed by a search of channels in order of access count as discussed above. The previously acquired channels can also be sorted by geographic search identifier so that previously acquired communications channels having a common geographic search identifier can be searched.
The methods and terminals of the present invention can be advantageously used in a CDMA communications system wherein an exhaustive search of CDMA channels by carrier frequency and PN sequence can be time consuming. Accordingly, each of the previously acquired communications channels can be a CDMA channel, so that the user terminal stores a carrier frequency and a PN sequence for each of the previously acquired communications channels. The user terminal can also store a base station identification and a base station location for each of previously acquired communications channels. Accordingly, the user terminal can have multiple previously acquired channel entries with the same carrier frequency and PN sequence but with different base station locations. In other words, the user terminal can distinguish different CDMA channels having the same carrier frequency and PN sequence by the locations of the transmitting base stations.
The methods of the present invention can also be applied to dual use terminals adapted for communications with both CDMA and non-CDMA communications systems. In particular, the user terminal searches for non-CDMA communications channels when CDMA communications channels are not available. The terminal can preferably provide CDMA communications when a CDMA channel is available and provide non-CDMA communications when a CDMA channel is not available. Furthermore, the dual use terminal can periodically search for available CDMA communications channel when providing non-CDMA communications so that CDMA communications can be provided if a CDMA channel becomes available.
According to the methods and terminals of the present invention, the CDMA channels which are most likely to be available are searched first thereby reducing search times. In particular, the search can be prioritized by the most recently acquired channel(s), by the most frequently acquired channel(s), and/or by the location of the previously acquired channel(s).